| Summary: | 博士 === 國立成功大學 === 微電子工程研究所 === 102 === In this study, research on application of modification layer in both small molecule and conjugated polymer in organic thin film transistors was presented.
For improvement of the insulator’s property to prevent unstable characteristics and breakdown phenomenon result due to the high leakage current, tri-layer [1 non-cross-linked PVP (NCPVP)/ 2 cross-linked PVP (CPVP)] dielectrics structure was applied to small molecule pentacene-based OTFTs. From the scanning electron microscope (SEM) results, some cracks and valleys couldn’t be found on the surface of PVP dielectric. This is because the NCPVP/2CPCP specific structure could not only strengthen the polymer insulator property, but also optimize the surface energy to facilitate the deposition of pentacene film. The experimental results suggest that the leakage current of the device with tri-layer NCPVP/2CPVP dielectrics has best leakage performance compared with single-layer CPVP or dual layer CPVP, which may be ascribed to the surface energy optimal matching and the reduction of cracks and valleys generated during the thermal treatment process. Therefore, the lower surface energy on the surface of tri-layer polymer dielectrics results in better pentacene film morphology and larger crystalline size, contributing to better output characteristics and more stable properties of OTFTs.
Except active film quality improvement, enhancing the injection efficiency from source/drain to active layer is another effective way to improve device performance. We proposed 1 nm of F4-TCNQ insertion between pentacene and Au source/drain, which is accompanied by improved mobility from the maximum 9 cm2/ Vs to 11.27 cm2/ Vs and reduced threshold voltage from -1.46V to -1.05V, and the contact resistance was also modified from 44 kΩ to 29.58 kΩ at VG=-3.6V. With optimal 1 nm-thick F4-TCNQ, work function modification by tunneling carrier transport will make the carrier injection process more easily. The experimental results suggest 1nm F4-TCNQ insertion also get smoother surface, which contribute to best performance.
For the polymer-based OTFTs, an n-type transistor with the composite channel structure consisting of Poly(styrenesulfonate)-polyvinyl alcohol (PSS-PVA) modification layer and active material polyaniline-polyvinyl alcohol (PANI-PVA) was proposed. From the experimental results observed, the PANI-PVA device without the electrolyte PSS-PVA exhibited a resistor behavior. With the insertion of the PSS-PVA electrolyte layer, the device behaved as a n-type transistor working in enhancement mode. This is due to the H+ cations of PSS-PVA inserted into PANI-PVA, which was then doped by H+ cations, and the PANI-PVA state changed from emeraldine base to emeraldine salt. However, PANI-PVA OTFTs exhibit n-type transistor behavior, which is available for the design of organic complementary metal oxide semiconductor (CMOS) integrated circuit. For PANI-PVA n-type transistor, the filed-effect mobility is 2.32 cm2/ Vs and on/off ratio is about 10.
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